Boost Surging / Fluctuation. What to try next?

[RickkyP]

So I've had boost surging / fluctuation with my 1.8T AUM and I can't seem to trace the problem.
It doesn't always happen and its always when near wide open throttle, sometimes it happens, other times it doesn't.

In the last year and a bit I've done:
- New Coil Packs + New loom wiring
- New MAF
- N249 + SAI Deleted
- PCV Pipes pretty much all changed.

So after all that the problem still remained and I had the error
17705 - Pressure Drop between Turbo and Throttle Valve (check D.V.!)

So I put it down a boost leak. Well today I made a boost leak tester using a tyre stem and baked beans can hooked up to a tyre inflator.
There are no leaks I could find in the system, when pressurised it slowly bled air into the engine and out through the dip stick hole, which I assume is either some air blowing past the turbo oil seals, or the PCV check valve? (I clamped off the pipe from the rocker to the TIP whilst testing)

Next I cleaned the throttle body as apparently this can cause surging as well, it was pretty clean, but I got it immaculate and then calibrated it using VCDS.
End result - Car still surging on boost.

What's next to try?

 

[Tomsy]

a new dv?

 

[RickkyP]

There was no obvious leaking from it on the boost test, and I also tried running the car with the feed into the top of it blanked off, effectively and it made no difference.

 

[ayfreetee]

I had this on my old aum a3, it turned out to be the N75 valve.

 

[RickkyP]

Thanks, was there a test procedure? Or did you just replace it and find out it worked after?

 

[ayfreetee]

Just bought another one.

 

[Stuart B]

I bought a replacement DV when I got that error - but I had also been swapping around N75's at the same time - whilst trying to resolve a high boost reading.